This invention relates to an electric power assist apparatus for motor vehicle steering.
A motor vehicle steering apparatus typically includes a steering gear connected to dirigible wheels of the motor vehicle, a steering shaft connected to the steering gear and rotatable on a steering column of the motor vehicle, and a steering hand wheel on the steering shaft where manual effort is applied to steer the motor vehicle by rotating the steering shaft. To reduce the manual effort required to rotate the steering shaft, the steering apparatus may further include an electric power assist apparatus including an electric motor and a speed reducer consisting of a housing attached to the steering column, a worm driven by the electric motor, and a worm gear on an output shaft of the speed reducer interposed between the steering shaft and the steering gear. Dimensional clearance between the worm and the worm gear necessary for low friction operation, even when optimized as described in U.S. Ser. No. 08/803,463, filed Feb. 20, 1997, and assigned to the assignee of this invention, may manifest itself when the electric motor is off as perceptible noise attributable to rattling of gear teeth on the worm gear against gear teeth on the worm induced by vibration of the speed reducer housing. An electric power assist apparatus according to this invention is a novel alternative to the electric power assist apparatus described in the aforesaid U.S. Ser. No. 08/803,463.
Gutmann, F. T., 18 Ways to Control Backlash In Gearing, Product Engineering (Oct. 26, 1959), pp. 71-75, discloses eighteen classic mechanisms for reducing backlash and rattle for a variety of gear configurations, including worm-gear and bevel-gear assemblies.
Enveloping gears are well known in the gear art and are discussed in detail in Townsend, D. P., Dudley""s Gear Handbook, Second Edition, McGraw Hill (1991), Chapters 2 and 4.
Involute curves and gears are also long known in the gear art and are discussed in detail in Lynwander, P., Gear Drive Systems, Marcel Dekker, Inc, (New York 1983), pp. 23 to 25.
Totten, D., GB 2,224,805A, entitled GEAR ANTI-RATTLE DEVICE, published May 16, 1990, describes a gear train anti-rattle device in the form of an elastomeric extension 21 applied to a standard steel gear, the extension having teeth of substantially the same profile, but slightly oversized relative to the gear teeth to occupy the backlash space between a pair of meshing gears. This elastomeric material acts to absorb vibrational energy and reduce or eliminate the impact noise normally developed when the gears are subjected to oscillatory torque and to deform the extension teeth when a predetermined level of mean torque is reached.
The major problem with solutions like Totten is that the plastic gear teeth are compressed or sheared, thereby resulting high wear and tear and rapid deterioration. This is not suitable for automotive applications that require components last about ten, if not twenty, years of nearly continuous use.
This invention is a new and improved electric power assist apparatus for motor vehicle power steering including an electric motor and a speed reducer. The speed reducer includes a worm driven by the electric motor and a molded plastic worm gear on an output shaft of the speed reducer having gear teeth meshing with gear teeth on the worm. The polymer from which the worm gear is molded has substantial structural rigidity, and the integrally molded gear teeth thereon are modified xe2x80x9cenvelopexe2x80x9d gear teeth each having a crest flush with an outer cylindrical wall of the worm and a pair of flanks which diverge from each other for maximum tooth rigidity from a narrow flat end in the plane of a first side of the worm gear to a wide flat end in the plane of a second side of the worm gear. A plastic tooth ring is insert molded on the worm gear over the narrow flat ends of the modified envelope gear teeth from a polymer having substantially greater resilience than the polymer from which the worm gear is molded. The plastic tooth ring includes a plurality of resiliently flexible gear teeth having flanks which circumferentially overlap the flanks of the modified envelope gear teeth. When the electric motor is off, the gear teeth on the worm mesh with the resiliently flexible gear teeth on the tooth ring with zero clearance therebetween to eliminate rattling between the gear teeth on the worm and on the worm gear. When the electric motor is on, the worm teeth induce resilient flexure of the flexible gear teeth on the tooth ring until the worm teeth engage the structurally more rigid modified envelope gear teeth on the worm gear for torque transfer between the worm and the worm gear.